Common processes for liquefying natural gas are cascade processes, mixed refrigerant processes and precooled mixed refrigerant processes. In cascade processes, the natural gas is cooled and liquefied by sequential heat exchange with a series of different refrigerants contained in separate refrigeration systems. The refrigerants are selected and arranged so that their composite cooling curve closely matches the cooling curve of the natural gas. Each individual refrigerant provides the cooling duty over its optimum range. By using a sequence of refrigerants, the feed stream of natural gas is cooled from around ambient temperature to about -265.degree. F. (-165.degree. C.), a typical temperature for liquefied natural gas.
Although cascade processes are thermodynamically efficient, they have the drawback of requiring a great deal of expensive equipment. Since each refrigerant is typically handled by a separate refrigeration system, many compressors and other components must be used. To overcome this problem, mixed refrigerant processes have been developed which approach the thermodynamic efficiency of cascade processes, but which require less equipment.
In mixed refrigerant processes, a mixed refrigerant composition is selected which has a cooling curve that closely matches the cooling curve of the natural gas. However, rather than being handled in separate refrigeration systems, the individual refrigerants are mixed together and are handled by one refrigeration system. The mixed refrigerant typically consists of several refrigerant components having different boiling points. The mixed refrigerant components having the higher boiling points are used to provide the initial cooling, and those having the lower boiling points are used to liquefy the natural gas. With the mixed refrigerant vaporizing at different temperatures and pressures, the components of the mixed refrigerant are able to provide staged coolings over their respective optimum temperature ranges.
LNG plants which employ mixed refrigerant processes generally cost less to build and operate than those using cascade processes. As mentioned, they cost less to build because only one refrigeration system is required. They cost less to operate due to the utilization of larger compressors which are mechanically more efficient than the multiple smaller compressors required for cascade processes.
A refinement on mixed refrigerant processes is the use of an additional refrigeration system to precool the natural gas prior to heat exchange with the mixed refrigerant. This additional refrigeration system can also be used to cool the mixed refrigerant. The additional refrigeration system can employ a single refrigerant or a multicomponent refrigerant. Such systems are known as precooled mixed refrigerant processes or as combined cascade and mixed refrigerant processes. U.S. Pat. No. 3,763,658 to Gaumer et al discloses a precooled mixed refrigerant process which utilizes a single-component precooling refrigerant. The single-component refrigerant can be a C.sub.2, C.sub.3 or C.sub.4 hydrocarbon. The mixed refrigerant is a four-component refrigerant consisting of nitrogen, methane, ethane and propane. An example of a precooled mixed refrigerant process which utilizes a multicomponent precooling refrigerant is described in U.S. Pat. No. 4,229,195 to Forg. That process uses a mixture of C.sub.2 and C.sub.3 hydrocarbons as the precooling refrigerant. The mixed refrigerant used to liquefy the natural gas consists of nitrogen, methane, ethylene and propane.
By using an additional refrigeration system to precool the natural gas and the mixed refrigerant, precooled mixed refrigerant processes can more closely match the cooling curve of the natural gas, thereby achieving a better thermodynamic efficiency.
Despite the efficiencies of current cascade, mixed refrigerant and precooled mixed refrigerant processes, none are thermodynamically efficient for the liquefaction of certain natural gas streams available at high pressure. The present invention is aimed at providing such a process.